Many areas of the United States (and other parts of the world) are crisscrossed by streams, creeks, brooks, sloughs, rivulets and other small waterways. These features are an important part of their local riparian ecosystems, but in many locations, the channels impede vehicular traffic. To address this problem, culverts—consisting frequently of a small-diameter steel, concrete or fiberglass/plastic tube—are installed to direct water under a road, railway bed or pedestrian crossing. FIG. 2 shows a typical half-pipe galvanized steel culvert: a corrugated steel sheet 200 is placed over a stream 210, and soil is placed over the sheet (220, generally) to build up the grade so that road 230 can cross.
Culverts are usually sized based on statistically-expected water flows based on known storm events to reduce the chance that they will be overfilled in a flood, but their size is often smaller than that of the natural stream bed, leading to an increase in water velocity and volume that frequently results in stream bank erosion at either side. In fact, culverts are often built with strong concrete entrances, as shown in FIG. 3, to protect the culvert from damage, but this often causes a “necking down” or “funneling” effect that threatens the banks upstream with erosion from flooding conditions and the bed and banks downstream of the culvert with scouring due to the increased velocity of the water exiting the culvert. Increased erosion can result, leading to recessed pools on the downstream side of a culvert (FIG. 4).
Culverts also adversely affect the passage of wildlife from one side to the other: terrestrial animals may have to walk over the road or track (rather than along the stream bank), and aquatic animals may be reluctant to swim through the culvert because it presents different and unexpected visual cues that inhibit them from entering. Further, if erosion and hydraulic scouring have caused the downstream portion of the waterway to recede from the culvert outlet, a condition called “perching,” it may be impossible for fish to leap up to the culvert on their journey upstream. With the increased water velocity resulting from the funneling of the waterway into a constrained pipe, migrating and spawning fish such as steelhead and salmon may find it impossible to swim against the current preventing further migration upstream. The effect on non-aquatic wildlife passage is to force them to cross the road resulting in increased risk of harm to animals and significant loss to vehicles and even human lives.
Alternative structures that address some of these shortcomings may be useful when existing culverts must be repaired or replaced, and when new stream crossings are to be built.